Clamping elements are generally known where the clamping force can be cancelled out so as to release the clamping element with the aid of a single-acting hydraulic cylinder. The way in which such a clamping element can operate is shown for example in U.S. Pat. No. 4,273,461. This patent specification also shows a typical sphere of application for a clamping element of the type which is the object of the present invention, namely to press two objects together with the possibility, when required, of being able to release the coupling between the two objects. In the said U.S. Pat. No. 4,273,461 the two objects comprise parts of a ball and socket joint. The areas of application for clamping elements of the type concerned can however be quite numerous and are in no way restricted to this application.
A typical feature of a clamping element as shown in the said U.S. Pat. No. 4,273,461 is that it is relatively large and complicated and hence both expensive to manufacture and in many cases difficult to apply.
A major contributory factor to the high production costs for conventional clamping elements of the type which includes single-acting hydraulic or pneumatic cylinders is that the cylinders have to be manufactured with high accuracy so that the requisite seal can be achieved between the cylinder wall and the piston in the cylinder. Normally sliding seals are certainly arranged between the moving parts, but these can not eliminate the need for extreme precision during the manufacture of the inside of the cylinder. One example of a clamping element of the type specified in the introduction is illustrated in German patent application No. 2,065,506, where an O-ring has been arranged in a known manner between a hydraulic piston and the inside of the cylinder. A similar arrangement is illustrated in U.S. Pat. No. 2,552,304. One way of reducing the manufacturing cost of the cylinders is, instead of sliding seals, to employ membrane seals, which are known in connection with membrane boxes especially for pneumatic coupling elements. An example of such an element is shown in German patent publication No. 2,129,750. However a limitation in such a design is that it can be employed mainly only for comparatively low clamping forces. Furthermore the clamping force must be brought about completely by separate organs, which naturally require space and render construction more expensive, so that they can become both unacceptably expensive and difficult to employ.
The object of the present invention is to provide a clamping element without the disadvantages and limitations which characterise earlier designs. More particularly, one aim is to provide a clamping element which consists of only a few components, none of which require especially high manufacturing precision.
It is also an aim to provide a clamping element which, in spite of relatively small external dimensions, can generate a high clamping force and which, because of this, can be employed in a number of different applications, for example for clamping two objects which are to be combined, for restraining work pieces, for setting elements in fixed positions etc.
These and other objectives can be achieved by virtue of the fact that at least to a considerable extent the spring return in the clamping element is brought about by means of a sleeve of rubber or rubber-like material which fills a gap between the piston and cylinder wall or between the piston and a lining arranged in the cylinder, whereby the rubber sleeve is firmly vulcanised, firmly glued or surface-fixed in some other manner both against the cylindrical exterior of the piston and against the cylinder wall or said lining along an axial length which considerably exceeds the thickness of the gap. The rubber sleeve can be responsible completely for the spring return, but it is also possible to supplement the rubber sleeve by one or more metal springs so as to provide the powerful spring action which is required for strong clamping.
In the case where the rubber sleeve is responsible completely for the spring return it is preferable that the rubber sleeve be firmly vulcanised or surface-fixed in some other manner against the piston and against the cylinder wall or against the said lining along an axial length which amounts to at least five times the width of the gap. If one or more metal springs are also provided to supplement the action of the rubber sleeve as return spring, the length of the rubber sleeve can be correspondingly reduced, e.g. to twice the width of the gap.
To further simplify and cheapen the manufacture of the clamping element in accordance with the invention the rubber sleeve can be vulcanised firmly in a metal sleeve, which is subsequently shrunk and fixed in the cylinder by gluing between the cylinder wall and the side of the metal sleeve.
In accordance with a preferred aspect of the invention, the piston rod is designed as a clamping screw. The pressure medium can be introduced into the pressure chamber through a channel in the piston rod/clamping screw. Naturally however it is also possible to introduce the pressure medium using a known method through an inlet aperture at the opposite end of the pressure chamber.
Further characteristics and aspects of the invention, also its advantages, will be illustrated by the patent claims which follow and by the following description of some preferred embodiments.